The invention relates to an apparatus for feeding sheets from a stack to a sheet-processing machine and to a method of determining the vertical position of stacked sheets using a sensor device.
Apparatuses for feeding sheets from a stack to a sheet-processing machine, for example, to a sheet-fed printing machine are known. In order to be able to ensure the exact and fault-free feeding of the sheets, the vertical position of the sheet stack, which can be moved vertically, must be set exactly within close limits. In order to determine the vertical position of the topmost sheet lying on the stack, Published German Patent Application DE-A 17 86 008 discloses a sheet feeding apparatus that uses a mechanical sensing device, which senses the top of the stack, and a photo electric sensing device. These optical sensors have the disadvantage that they are very sensitive to dirt and therefore are only reliable to a certain extent. In addition, fluttering sheets, which occur when air is blown under the topmost sheet in order to loosen the sheet stack, lead to inaccurate measurement. The optical sensors also have the disadvantage that, because of their overall height, they cannot be used in every case. A further disadvantage is the unreliability of optical sensors when using exotic printing materials, such as transparent, black or metallic printing materials.
It is accordingly an object of the invention to provide an apparatus and a method for determining the position of the sheet stack which overcomes the above-mentioned disadvantageous of the prior art apparatus and methods of this general type. In particular, it is an object of the invention to provide such an apparatus and a method with which the vertical position of the sheet stack can be determined and then set very exactly and with high reliability.
With the foregoing and other objects in view there is provided, in accordance with the invention, an apparatus for feeding sheets from a stack to a sheet processing machine, in which the apparatus includes a sensor device for determining the position of the topmost sheet lying on the stack. The sensor device for determining the vertical position of the topmost sheet lying on the stack includes at least one ultrasonic sensor. This ultrasonic sensor transmits ultrasonic pulses which are directed onto the sheet stack and are reflected as an echo to a sensor belonging to the ultrasonic sensor. Using an additional device, an ultrasonic propagation time measurement is carried out, and the result of an evaluation is transmitted to a control unit, which drives a vertical adjusting device for the sheet stack in order to raise and lower the latter. Using the ultrasonic sensor, very accurate and reliable determination of the vertical position of the sheet stack is possible, so that the latter can be tracked, cyclically or continuously, in such a way that the topmost sheet on the stack can be aligned in the desired manner opposite a transport element used for the onward transport of the sheets to be separated. High functional reliability for the feed apparatus can therefore be ensured. The ultrasonic sensor is considerably less sensitive to dirt than known optical sensors. In addition, it is advantageous that a compact, preferably miniaturized ultrasonic sensor can be used, which can be arranged at virtually any point around the sheet stack. A further advantage is that the sensor has the same sensitivity to all grades of printing material.
In accordance with an added feature of the invention, the ultrasonic sensor is arranged below the upper edge of the stack, that is to say the ultrasonic pulses emitted by the sensor strike at least one of the ends of the sheet stack at an angle. This permits the evaluation of the differential propagation times of the echo reflected from the sheet stack and, if necessary, from the outgoing sheet or another reference point.
In accordance with an additional feature of the invention, the ultrasonic sensor is arranged in front of the side of the stack that faces the sheet-processing machine. The ultrasonic sensor is therefore associated with the leading area of the sheet stackxe2x80x94as viewed in the transport direction of the sheetsxe2x80x94and applies ultrasonic pulses to the leading edges of the sheets. In this case, therefore, the vertical adjustment of the sheet stack is performed as a function of the determined position of the upper edge of the stack at the leading edges of the sheets. As a result, exact and reproducible alignment of the topmost sheet on the stack with respect to the transport element used for the onward transport is possible.
With the foregoing and other objects in view there is also provided, in accordance with the invention, a method of determining the vertical position of stacked sheets, which includes the following steps: providing a sheet stack having a longitudinal side; providing ultrasonic pulses that strike the longitudinal side of the stack at an angle; and performing an ultrasonic propagation time measurement to determine a position of an upper edge of the stack.
The method, which is possible because of the specific spatial arrangement of the at least one ultrasonic sensor opposite the sheet stack, is distinguished by high functional reliability and accuracy. Exact measurement of the vertical position of the topmost sheet of the stack can be ensured even during a fluttering movement of the sheet when, for example, the topmost sheet is being loosened with the aid of a gaseous medium, that is to say it is being lifted off the sheet stack. This applies irrespective of the reflectance of the sheet or of the sheet material. This means that the method can be used universally, for example for paper sheets, transparent films, metallic or metallised sheets.
In accordance with an added mode of the invention, the differential propagation times between the echo from the sheet leaving the stack and the echo from the topmost sheet lying on the stack are evaluated. The ultrasonic pulse or the ultrasonic pulse train that is transmitted by the ultrasonic sensor runs from below against the stack and against the sheet that is being lifted off the stack and transported onward in the direction of the machine, and the pulse or pulse train is reflected. A large echo is reflected from the outgoing sheet, and many small echoes are reflected from the leading edges of the stacked sheets and these reflected echoes are obtained by the receiver belonging to the ultrasonic sensor. The echo arriving last at the receiver is that from the topmost sheet which, as compared with the other small echoes, which originate from the sheets located below it on the stack, has the longest path. It is therefore readily possible to distinguish between the echoes from the outgoing sheet, the topmost sheet on the stack, and the remaining stacked sheets. The position of the topmost sheet lying on the stack that has been determined in this manner is compared with a desired vertical position of the sheet stack. In the event of any violation of an upper or lower limit, the sheet stack is moved vertically into a desired position with the aid of a vertical adjusting device, which is driven by a control unit.
In accordance with an additional mode of the invention, the differential propagation times between the echo from the topmost sheet on the stack and the echo from a fixed reference point are determined. The reference point, located within the feeder, can be formed, for example, by a reference web on a flat shaft which is used to align the leading edges of the sheets and to guide the sheets. Here too, because of the specific spatial arrangement of the ultrasonic sensor opposite the sheet stack, the ultrasonic pulses that are transmitted by the at least one ultrasonic sensor run both against the reference point and against the stack, and are reflected and passed as an echo to the receiver. The echo returned from the reference point is more powerful than the many small echoes which are generated by the sheet stack or by the leading edges of the stacked sheets.
In accordance with another mode of the invention, the differential propagation times between the echo from the topmost, loosened sheet, and the echo from the remaining sheet stack located underneath the loosened sheet are evaluated. Loosening of the topmost sheet is carried out with the aid of a gaseous medium or, if appropriate, by means of a mechanical device, as a result of which the topmost sheet executes a fluttering movement. The sheets located underneath it are at the same time, at least substantially, properly stacked. This mode and the immediately previously mentioned mode of the method permit the continuous determination of the vertical position of the topmost sheet of the stack, since, in order to evaluate the differential propagation times, an outgoing sheet, that is to say one lifted off the sheet stack and already partially transported away from the stack in the direction of the machine, is not needed for this purpose.
With the foregoing and other objects in view there is also provided, in accordance with the invention, a method of determining the vertical position of stacked sheets, which includes the following steps: providing a sheet stack having a longitudinal side; from a plurality of locations, transmitting and focusing ultrasonic pulses at point on the longitudinal side of the stack; and controlling the transmitted ultrasonic pulses such that the point, at which the transmitted ultrasonic pulses focus, is moved along the longitudinal side of the stack. As a result of the focussing of the ultrasonic pulses, very good signal quality with high intensity can be implemented, which improves the functional reliability and the accuracy of the stack vertical position determination.
In accordance with a further mode of the invention, provision is made for the focus of the sound to be moved in a line at right angles to the leading edges of the stacked sheets. In this case, a scanning plane is scanned in which the leading edges of the stacked sheets are preferably arranged. The scanning plane is therefore, so to speak, scanned until the topmost point of the stack has been found. The displacement of the focus, that is to say the distance which the focus of the sound has traced, permits the inference and therefore the determination of the actual position of the topmost sheet of the stack relative to a desired intended vertical position of the stack, for example relative to a transport element for the onward transport of the sheets into the machine.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an apparatus for feeding sheets, and method of controlling the vertical position of stacked sheets, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.